1. Field of the Invention
This invention relates to a novel developer and improved method for developing positive photoresist layers, and more particularly to a developer used in the processing of alkali soluble resin--diazo ketone photoresists to increase the contrast of the developed photoresists. The invention is especially useful because it results in a higher contrast that renders the adverse effects of any exposure differences which occur less significant, i.e., the higher the contrast, the less affected are the geometry dimensions in the patterned photoresist by exposure variations; provided, of course, that the exposure is adequate to expose the photoresist.
2. Description of the Prior Art
Photoresists are materials which change their solubility response to a developer solution after a film of the photoresist has been applied to a surface and exposed to an irradiation source, such as to ultraviolet light. As a consequence of the exposure, a different solubility rate results between the exposed and unexposed (masked over) portions of the photoresist film that yields a surface relief pattern after the development of the film. Those photoresists which become more soluble in the exposed regions are referred to as positive photoresists. However, because the alteration of the solubility of the photoresist is only a relative change and even the less soluble unexposed portions of the photoresist dissolve to some extent, any process which enhances the developing rate difference (typically called contrast) between the relatively soluble and relatively insoluble photoresist portion is advantageous.
Positive photoresists are typically comprised of an aqueous alkaline soluble resin, such as novolak resin or poly(p-hydroxystyrene), and a diazonaphthoquinone sulfonic acid ester sensitizer. The resin and sensitizer may be applied by a method such as spin coating from an organic solvent or solvent mixture onto a substrate, such as silicon wafers and chrome plated glass plates. Developers that have been used to process the positive photoresists are aqueous alkaline solutions, such as sodium silicate, potassium hydroxide, sodium hydroxide, tetramethyl ammonium hydroxide and ammonium hydroxide. The developer removes the areas of the coated photoresist film that have been exposed to light or other form of irradiation so as to produce a pattern in the photoresist film.
The existing positive photoresist systems can have a contrast (gamma) of three (3) to five (5) depending upon process conditions. The problems associated with a gamma of this order are evident in the reported performance. Typically the higher gammas of five are obtained by using a weak developer and a prolonged developing with a resulting loss of sensitivity in order to control the process. It is thus apparent that a need exists for a developer which is capable of providing a gamma of the order of at least 5 and preferably of the order of 10 or greater without sacrificing sensitivity.